1. Field of the Invention
An aspect of this disclosure relates to a thermal printer.
2. Description of the Related Art
A known thermal printer includes multiple heating elements that generate heat corresponding to the amounts of energy applied, and forms a multi-gradation image on a recording medium.
In such a thermal printer, for example, gradation levels are determined based on a relationship, which is indicated by FIG. 17, between the optical density of a printed image and the energy applied to the heating elements, such that differences in optical density between the gradation levels become substantially the same, and the amounts of energy applied to the heating elements are set for the respective gradation levels.
Also, Japanese Laid-Open Patent Publication No. 04-220358, for example, discloses a thermal printer where the amounts of energy applied to heating elements are determined based on linear approximation of the relationship between the optical density of a printed image in a medium density range and the applied energy, in order to reduce the processing load.
The relationship between the optical density and the reflectance indicating brightness of a printed image is represented by a formula below.Optical density=−log(reflectance)
Accordingly, as illustrated by FIG. 18, the reflectance changes sharply in a low optical density range and changes gradually in a high optical density range. For this reason, even when the amounts of energy applied to heating elements are determined such that the optical density changes at a constant interval as illustrated by FIG. 17, changes in reflectance in a high density range may become small and the gradation reproducibility may become low.
FIGS. 19A and 19B illustrate exemplary printed images. FIG. 19A is a printed image printed by applying energy to heating elements at levels that are determined based on the relationship between the optical density and the energy illustrated by FIG. 17 such that changes in optical density between gradation levels become substantially the same. FIG. 19B is an image printed by applying energy to heating elements at levels that are determined based on linear approximation of the relationship between the optical density and the energy applied to the heating elements.
When the amounts of energy applied to heating elements are determined based on the optical density, the reflectance of a printed image in the low density range sharply changes, and gradations of the printed image in the high density range become indiscernible. This in turn may practically reduce the number of reproducible gradation levels.